A multifaceted program to examine the biochemistry and physiology of mononuclear phagocytes from man and animals. particular emphasis is placed upon an understanding of the plasma membrane in terms of surface receptors, mechanisms of membrane fusion and interiorization and its intrinsic polypeptides and lipids. Studies continue on the Secretory Role of Macrophages and the Identification of Secreted Enzymes and Factors. In addition, the characterization of adherent cell populations from lymphoid tissue and their role in the immune response is in progress.